1. Field of the Invention
The present invention generally relates to a transmission and reception apparatus and method for use in a wireless communication system. More particularly, the present invention relates to an apparatus and method for adaptively transmitting and receiving a signal according to a channel state or condition in a wireless communication system.
2. Description of the Related Art
A wireless communication system transmits and receives a signal through a radio channel between a transmitter and a receiver. The transmitter modulates data into a signal through which wireless communication is enabled, and outputs the modulated data. The receiver receives and demodulates a modulated signal transmitted through the radio channel.
The above-described wireless communication system controls bit and power allocation for a transmission signal to maximize the channel capacity between the transmitter and the receiver. Bits of the transmission signal represent bits of a transmission signal per modulation symbol, and differ according to modulation schemes such as binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), and so on. The modulation schemes are adaptively changed according to a radio channel state or condition.
Accordingly, the transmitter and receiver must have knowledge of channel state information (CSI). However, research on adaptive resource allocation based on the CSI has been focused on a downlink of the wireless communication system because the downlink requires a higher transmission rate than an uplink. For a better understanding of the present invention, it is assumed that the transmitter is a base station (BS) and the receiver is a user.
When the BS obtains the CSI, time division duplexing (TDD) and frequency division duplexing (FDD) operate differently.
TDD is a scheme in which the uplink and downlink share the same channel on the basis of time division, and FDD is a scheme in which the uplink and downlink use different channels regardless of time. Because channel characteristics are the same between the downlink and uplink in the TDD scheme, the BS measures CSI of the uplink and regards the measured CSI as CSI of the downlink if a channel state is the same during one frame.
In the case of FDD, the downlink and uplink have different channel states due to a large frequency interval therebetween. Accordingly, the user measures the CSI of the downlink and then feeds back the measured CSI to the BS. However, in a process in which the user feeds back the CSI, performance degradation occurs due to the feedback overhead, feedback delay, and feedback error. In the process of acquiring the CSI, it can be seen that the TDD system is better than the FDD system.
On the other hand, a cellular wireless communication system achieves high frequency efficiency through “frequency reuse”. The term “frequency reuse” refers to the same frequency being used between cells that are not adjacent to each other. When a frequency reuse level is high, the frequency efficiency increases. However, a problem occurs in that the same channel interference from adjacent cells becomes severe. Accordingly, adaptive resource allocation applied to the cellular wireless communication system in which the frequency reuse level is high is based on a signal power to interference power ratio (SIR) instead of CSI. Because interference power in the SIR depends upon the user's location, the BS cannot measure downlink interference power. Therefore, even when the TDD system is used in the cellular wireless communication system in which the frequency reuse level is high, the user needs to measure a downlink SIR and feed back the measured SIR to the BS. However, there is a problem in that performance is degraded due to the feedback overhead, feedback delay, and feedback error in the feedback process as described above.
FIG. 1 is a block diagram illustrating a conventional adaptive transmission and reception apparatus operating on the basis of a SIR in a wireless communication system in which the same channel interference is present on an uplink and a downlink channel. In FIG. 1, the solid line indicates a signal flow between an adaptive transmitter and an adaptive receiver. Here, a signal is transmitted through a wireless interface.
Referring to FIG. 1, the adaptive transmitter 110 adaptively transmits information to the downlink/uplink on the basis of a SIR measure prior to feedback from a channel estimator/SIR measurer 120. The channel estimator/SIR measurer 120 measures a downlink/uplink SIR using a reference signal such as a pilot signal among downlink/uplink reception signals, and feeds back the measured SIR to the adaptive transmitter 110. The adaptive receiver 130 adaptively receives a downlink/uplink signal using a channel estimate output from the channel estimator/SIR measurer 120.
The representative technology capable of increasing the downlink capacity through adaptive resource allocation based on a SIR in the wireless communication system in which the same channel interference is present is referred to as an orthogonal frequency division multiple access (OFDMA) scheme using adaptive subchannel, bit and power allocation.
When multiple subcarriers are combined in the conventional OFDMA system, the combined subcarriers are referred to as a subchannel serving as a basic unit of user data mapping. The BS transmits a unique pilot symbol of a cell or sector at a uniform subcarrier and/or symbol interval. Each user measures a SIR value for an associated subchannel using pilot symbols positioned within a predetermined duration of each subchannel and feeds back the measured SIR value to the BS. The BS performs the adaptive subchannel, bit and power allocation using fed-back SIR measures for subchannels of all users. The BS maps, adaptively modulates, and transmits user data according to a result of the allocation, and notifies all user terminals of the allocation result. Each user terminal demodulates and extracts its own data according to the notified allocation result.
FIG. 2 is a block diagram illustrating a conventional downlink OFDMA apparatus for performing the adaptive subchannel, bit and power allocation.
Referring to FIG. 2, K users 240 receive a signal transmitted from the BS through K unique selective fading channels 230.
Referring to an internal structure of the BS 200, a subchannel, bit and power allocator 210 receives required transmission rates of all users, such as user transmission rates, every allocation period, and receives SIR measures for subchannels of all users every SIR measurement period. Then, the subchannel, bit and power allocator 210 performs the adaptive subchannel, bit and power allocation every allocation period, and outputs a result of the allocation to a subchannel, bit and power mapper 212. The subchannel, bit and power mapper 212 receives the allocation result output from the subchannel, bit and power allocator 210, and arranges all input user data, signaling data, and pilot symbols in temporal frequency gratings. The signaling data includes the allocation result such that the allocation result can be transferred to all users.
A predetermined number N of adaptive modulators 214 modulate the user data, signaling data, and pilot symbols using a modulation scheme associated with the number of allocated bits at a power allocated every subchannel and output modulation results to an inverse fast Fourier transform (IFFT) processor 216. The IFFT processor 216 transforms all subchannel modulation signals input from the adaptive modulators 214 in a parallel fashion according to the IFFT result. A parallel-to-serial (P/S) converter 218 converts parallel outputs of the IFFT processor 216 into a serial signal. A guard interval (GI) inserter 220 inserts a GI into the signal output from the P/S converter 218 and outputs a result of the insertion. The GI is used to mitigate inter-symbol interference (ISI) that occur due to the multipath delay when a signal goes through a radio channel. This is a well-known benefit of OFDM modulation. A signal output from the GI inserter 220 is transmitted to user terminals 240 through radio channels 230 corresponding to users.
An internal structure of the K-th user terminal 250 of the user terminals 240 is illustrated in FIG. 2.
A GI remover 252 removes a GI from a received signal and outputs a result of the removal. A serial-to-parallel (S/P) converter 254 converts an output signal of the GI remover 252 into parallel signals. A fast Fourier transform (FFT) processor 256 transforms the parallel signals output from the S/P converter 254 according to the FFT result. A predetermined number of adaptive demodulators 258 extract an allocation result from signaling data and perform adaptive demodulation using a channel estimate obtained from a channel estimator/SIR measurer 262 according to the number of bits of each subchannel allocated to User K.
A bit extractor 260 collects demodulated data of subchannels, allocated to User K, output from the adaptive demodulators 258. The user terminal 250 includes the channel estimator/SIR measurer 262. The channel estimator/SIR measurer 262 of the user terminal 250 receives signals output from the FFT processor 256, and obtains channel estimates and SIR measures for all subchannels. Then, the channel estimates obtained from the channel estimator/SIR measurer are transferred to the adaptive demodulators. Then, the SIR measures are fed back to the BS through a wireless process. To improve downlink performance through adaptive transmission and reception in a wireless communication system in which the same channel interference is present, the user measures a downlink SIR regardless of the above-described TDD and FDD and feeds back the measured SIR to the BS. However, a problem of performance degradation due to feedback overhead, feedback delay, and feedback error occurs in the feedback process.
Table 1 shows feedback overheads for the number of various users, the number of subchannels, and SIR measures.
TABLE 1RCHK = 8 & N = 15K = 16 & N = 54250180 kbps1296 kbps500360 kbps2592 kbps
In Table 1, RCH, K, and N denote a SIR measure, the number of users per cell, and the number of subchannels, respectively. The feedback overhead is computed by Equation (1).OverheadFB=RCH×K×N×6 (bps)  Equation (1)
In Equation (1), “6” is the number of bits representing a SIR for one subchannel. The feedback overhead increases in proportion to a SIR measure, the number of users per cell, and the number of subchannels, and significantly reduces uplink throughput. There is a problem in that the feedback delay limits an application range of an adaptive transmission and reception scheme at a low data transmission rate.